An inverter connected on the input side to a generator and on the output side to a multiphase AC grid and with a switch between a midpoint of a DC intermediate circuit and a terminal for a neutral conductor of an AC grid is known from EP 2608375 A2. Whenever electric power is fed from a photovoltaic generator connected to the DC intermediate circuit into the AC grid by this known inverter, the switch is closed. The switch is an electromechanical switch connected in series with a further electromechanical switch. Such series arrangements of two electromechanical switches are also provided between an inverter bridge of the inverter and the terminals for the phase conductors of the AC grid. In order to test the electromechanical switches between the midpoint of the DC intermediate circuit and the terminal for the neutral conductor with respect to their separation performance, the switches may be opened and closed individually.
Even for testing the electromechanical switches, no state of the inverter in which all electromechanical switches between the inverter bridge and the terminals for the phase conductors of the AC grid are closed, while one or both electromagnetic switches between the midpoint of the DC intermediate circuit and the terminal for the neutral conductor is or are closed, respectively, is described in EP 2608375 A2.
From EP 2107672 A1 a three-phase inverter connected on the input side to a generator and on the output side to a multiphase AC grid without connection between the neutral line of the AC grid and the midpoint of the DC intermediate circuit is known. This inverter has a high efficiency, and it is functioning at a low intermediate circuit voltage in the DC intermediate circuit. However, it turns out that in this inverter a leakage current flowing from a generator connected to the DC intermediate circuit is relatively large. In addition, no asymmetric feed of electricity into the different phases of the AC grid is possible.
In the three-phase inverter known from EP 2107672 A1, the midpoint of the DC intermediate circuit is connected via filter capacitors to lines leading from the midpoints of the half-bridges of the inverter bridge of the inverter to the phase lines of the grid and with filter inductances being arranged in these lines. The half-bridges are configured as multi-level circuits, in particular as three-level circuits. The midpoints of the inverter bridges are each connected to the midpoint of the DC intermediate circuit via a bidirectional switch composed of two series-connected semiconductor switches with opposite blocking directions.
From EP 2375552 A1 an operating method of an inverter as described in EP 2107672 A2 and connected on the input side to a generator and on the output side to a multiphase AC grid is known. In this known method, the leakage current occurring during operation of the inverter is reduced by increasing the intermediate circuit voltage of the DC intermediate circuit on the input side of the inverter. This is possible in the range of low intermediate voltages, which is permitted by an inverter without connection between the midpoint of its DC intermediate circuit and the neutral line of the AC grid connected on the output side. In this voltage range between the peak value of the phase-to-phase line voltage of the AC grid and the double value of the peak phase-to-neutral line voltage of the AC grid, the switches of the inverter bridge are driven such that the AC currents fed into the phase lines are formed by so-called overmodulation. This overmodulation results in a leakage current that increases with increasing degree of overmodulation at decreasing intermediate circuit voltage. Accordingly, the leakage current, which further depends on the leakage capacitance of the generator connected to the DC intermediate circuit, may be reduced by increasing the intermediate circuit voltage.
From EP 2367272 A2 an inverter connected on the input side to a generator and comprising a three-phase bridge circuit is known, wherein phase lines at the output side of the inverter may be connected to a three-phase AC grid. The inverter comprises no connection to the neutral line of the AC grid. A capacitor is connected to each phase line, respectively, the capacitors being connected to a common connection point. An intermediate circuit on the input side of the inverter is connected to a serial arrangement of balancing capacitors being connected to the connection point via a damping resistor.
From US 2013/0229837 A1 a transformerless converter is known that is operated as an active filter to improve the power quality of an AC grid. The converter comprises a three-phase converter bridge connected on the AC side to the AC grid and on the DC side to a DC intermediate circuit. A controllable switch is provided between a midpoint of the DC intermediate circuit and a terminal for a neutral conductor of the multi-phase AC grid. The switch is combined with two separate choke modules in two branches of the neutral line connected to the midpoint of the actual DC intermediate circuit on the one hand and to the center point of an auxiliary capacitor module on the other hand. The converter may be configured by the switch to be either in a 3p3w configuration at open switch or in a 3p4w configuration at closed switch. The auxiliary capacitor module, which increases the capacity of the DC intermediate circuit in order to suppress ripple currents, forms part of the 3p4w structure then.